Hot melt pressure sensitive adhesives are usually based on a block copolymer such as styrene-ethylene/butylene-styrene, styrene-ethylene/propylene-styrene, styrene-isoprene-styrene and styrene-butadiene-styrene in combination with a tackifying resin and a plasticizing oil. Hot melt pressure sensitive adhesives can be divided into four general categories including permanent, semi-permanent, removable and freezer categories, each category having different adhesive characteristics. Permanent adhesives are designed to permanently adhere one substrate to another and provide either substrate failing or substrate distorting bonds. Semi-permanent adhesives are designed to be repositionable for a short period of time after application of one substrate to another. Freezer grade adhesives are designed to have good adhesion to substrates at temperatures of about -30.degree. C.
Removable hot melt pressure sensitive adhesives are designed to allow separation of substrates at any time after application of one substrate to another without substrate failure or adhesive transfer. These adhesives are generally characterized as having low 180.degree. peel values and high initial tack. These properties are typically achieved through the use of high levels of block copolymers, low levels of tackifying resin and high levels of a plasticizing oil. The block copolymer provides increased flexibility and a smooth, low peel value. The tackifying resin provides high tack, high adhesion and reduces viscosity. The plasticizing oil reduces peel values, viscosity and glass transition temperatures, increases flexibility and decreases the storage modulus.
To assure that the adhesive does not adhere too aggressively to the substrate and therefore assure its removability, it is necessary to utilize low levels of tackifying resin. This means that either the level of block copolymer or the level of oil must be increased. Increasing the concentration of block copolymer results in an increase in viscosity, whereas increasing the concentration of oil results in a product which stains the substrate.
U.S. Pat. No. 4,286,077 to St. Clair et al. issued Aug. 25, 1981 teaches a weather resistant hot melt adhesive which is a blend of a styrene-ethylene/butylene-styrene A-B block copolymer and a multiblock copolymer, namely Kraton.RTM. G 1657, which has a diblock content of about 35%, and a tackifying resin. Those formulas exemplified contain 10% by weight or less of a plasticizing oil, resulting in compositions which are permanent grade hot melt pressure sensitive adhesives having 180.degree. peel values to glass of greater than about 3 pounds per linear inch and very high viscosities. Of the compositions exemplified, the lowest viscosity is about 25,000 cPs at 177.degree. C.
The block copolymers provide a medium for dissolution or suspension of the tackifying resin and the plasticizing oil. A hot melt pressure sensitive adhesive utilizing the proper amount of a plasticizer should retain a solid physical nature upon aging while having significant flexibility. However, the block copolymers will reach a point at which they can retain no more oil. The oil will then separate from the adhesive and bleed into the substrate causing staining of the substrate. Increasing the levels of block copolymer or increasing the molecular weight of the block copolymer typically allows for the utilization of higher levels of oil which is necessary when compounding a removable hot melt pressure sensitive adhesive.
There remains a need in the industry for a removable hot melt pressure sensitive adhesive that is low in viscosity and which does not stain the substrates to which it is applied. The current inventors have discovered a removable hot melt pressure sensitive adhesive composition which allows for very low viscosities and does not stain a substrate upon aging, even at high temperatures.